core: Don't call unlink() on directory if it already exists
core: Don't call unlink() on directory if it already exists

If we fail to make a directory because it exists, we shouldn't call
unlink() on it.

sujitfulse/ostree,gatispaeglis/ostree,sujitfulse/ostree,gatispaeglis/ostree,sujitfulse/ostree,sujitfulse/ostree,resin-io/ostree,leandrosansilva/ostree,gatispaeglis/ostree,gatispaeglis/ostree,sujitfulse/ostree,gatispaeglis/ostree,GNOME/ostree,resin-io/ostree,gatispaeglis/ostree,leandrosansilva/ostree,GNOME/ostree,leandrosansilva/ostree,resin-io/ostree,GNOME/ostree,GNOME/ostree,leandrosansilva/ostree,GNOME/ostree,resin-io/ostree,resin-io/ostree,leandrosansilva/ostree